This invention relates to an airborne electromagnetic system of a kind which is suitable for geophysical prospecting using electromagnetic fields.
In electromagnetic prospecting a primary alternating electromagnetic field is transmitted from one point and the resultant field, (i.e. the primary field plus the secondary field from the earth), is received at a second point, which is spaced from the first point. Anomalies in the earth, e.g. conductive substances, can be detected due to the secondary fields which are generated in them by the primary field and which are received at the second point.
In order to measure the secondary field accurately it is important that the primary field at the receiver should be as constant as possible. Generally, for large scale towed bird systems, the primary field is produced by a transmitting coil in an aircraft and the primary plus the secondary field from the earth is measured at the centre of a receiving coil or coils in a bird which is towed by the aircraft. In a system which is towed by a helicopter, the primary field at the receiver is kept as constant as possible by rigidly mounting the receiver and transmitter to a relatively short rigid boom.
It is apparent that aerodynamic stability of the bird is highly desirable. A large transmitting antenna or loop is also desirable and, where necessary, it should be possible to space the transmitting and receiving coils from each other by a significant distance.